Steam power plants serve to generate electrical energy. They may take the form of thermal power plants, wherein the steam is generated using a fossil fuel-fired boiler. A thermal power plant substantially comprises in this case, in addition to the fired boiler, a steam turbine, a water-steam circuit and a condenser.
Steam power plants may likewise take the form of combined cycle gas turbine (CCGT) plants. Such combined cycle gas turbine plants in this case comprise at least one gas turbine, a water-steam circuit, a steam generator, a steam turbine, a generator and a condenser.
Steam power plants may also take the form of concentrated solar power (CSP) plants. Such thermal power plants in this case comprise at least the sun as heat producer, oil as transfer medium, a water-steam circuit, a steam generator, a steam turbine, a generator and a condenser.
The working medium used in steam power plants is deionized water, which is evaporated in the steam generator. The steam generated is passed into the steam turbine and expanded therein. The energy released on expansion is transferred via a shaft to the generator. The expanded steam is then fed to a condenser, and the liquid phase is condensed.
To assist the condensation process, an evacuation system is connected to the condenser, which system produces a vacuum in the condenser when the steam power plant is started up, and maintains it during operation. Steam turbine efficiency is increased by the vacuum and non-condensable gases are removed thereby from the liquid stream.
During the energy generation process various contaminants may be introduced into the working medium. In addition, various substances are added to the working medium for conditioning or purification. The working medium contaminated by contaminants or additives must be removed from the water-steam circuit as process wastewater, since the contaminants stand in the way of direct re-use as a working medium in the water-steam circuit.
Ammonia serves as an alkalizing agent for conditioning the feed water. Through the addition of ammonia, an increase in the pH value of the working medium may be achieved, whereby the relative corrosion rate of the feed water is reduced. Since the distribution coefficient of ammonia in liquids and steam is different, locally markedly elevated ammonia concentrations may occur in system parts involving evaporation and condensation processes (for example in the condenser, the evacuation system and the drum blowdown).
Process wastewaters arise at various points in the water-steam circuit. The process wastewater arising during production of the deionized water in a deionization plant is regeneration wastewater, which makes up the majority of contaminated process wastewater. On startup and shutdown, deficiencies (due to additional feeding of working medium) and excesses (due to discharge of working medium) of the working medium must be compensated. Moreover, process wastewaters arise due to continuous sampling and leaks in the water-steam circuit. As a result of the above-stated water losses, the water-steam circuit must be fed continuously with deionized water (DIW). Backwashing and regeneration processes in the deionization plant and condensate purification also result in process wastewater.
By way of example, a 400 MW combined cycle gas turbine plant produces annually around 14,000 tonnes of process wastewater in base-load operation in the case of steam generators with once-through boilers, and around 22,000 tonnes with circulating boilers. Hitherto, the majority of these process wastewaters has been discarded.
An exemplary 2×1050 MW fossil fuel-fired thermal power plant with a natural draft cooling tower and a wet limestone flue gas purification system produces as much as up to 100,000 tonnes of process wastewater per year in base-load operation, which has to be discharged into public water systems. Just about half of this amount is accounted for by the cooling tower.
Due to ever more stringent environmental legislation, and for countries suffering water shortages, reducing water consumption and thus reusing wastewaters and process waters within the water-steam circuit is becoming increasingly significant. In particular, the guidelines for discharging wastewaters into public water systems are becoming ever more stringent. The water consumption of a steam power plant should therefore be reduced as much as possible.
Contaminated process wastewaters arise for example in the steam drums of a combined cycle gas turbine plant. A plurality of steam drums at different pressure levels are conventionally provided. The combined cycle gas turbine plant may also comprise one or more “once-through” steam generators, which are also known as Benson boilers and which are in the main integrated into the high pressure stage. Because saturated steam is withdrawn from the steam drums, nonvolatile substances remain in the steam drums. These non-volatile contaminants are concentrated in the steam drums, and have therefore to be removed from the circuit by blowdown. Water is then lost from the circuit, which has to be compensated again by make-up water or “DIW”.
EP 1 706 188 B1 describes a method for recovering at least some of the blowdown water from a steam power plant. To reduce energy and water losses, the blowdown water from the high-pressure steam drum is conveyed on into the lower-pressure steam drum and expanded (“boiler cascading blowdown”). A disadvantage here, however, is that all the contaminants are passed from one pressure level to the next. It is therefore proposed that the steam separated off during high-pressure water-steam separation is fed to a lower-pressure steam drum (advanced cascading blowdown). This allows good energy utilization and re-use of at least some of the steam in the water-steam circuit. The remaining blowdown water must, however, be disposed of completely.
Further process wastewaters arise through drainage. Drainage is performed for example during ongoing operation from pipes which have been closed for a relatively long period in which condensate has collected. To this end, the pipes are opened briefly and thus drained. Water is then lost from the water circuit, which has to be replaced by make-up water (DIW). Drainage also in particular arises to a greater extent on startup and shutdown of the steam power plant, since for example on shutdown of the thermal power plant the steam in the water circuit gradually condenses and the resultant liquid water must not be allowed to stand in the plant parts, in particular in the heating surfaces. On shutdown, more water is drained from the water circuit than is replenished, until in the end no more water is replenished.
EP 1 662 096 A1 and U.S. Pat. No. 7,487,604 B2 each describe a method which makes it possible to recover drainage water from the steam power plant. The drainage water may be collected and combined, and also in part stored temporarily in a tank. The stored drainage water is then discharged into the environment by way of a pump. The tank here serves to reduce the running time and the interval frequency of the pump. The drained water may also be expanded in a separation vessel, to separate water and steam from one another. The separated steam and the drained DIW is then output into the environment. It is therefore proposed to collect all the drained water from at least one pressure stage of the water-steam circuit and store it, and to return the drained water collected and stored in this way substantially completely back to the water circuit via a water treatment plant.
However, no method has hitherto been known from the prior art by which all the process wastewaters from a power plant can be collected and largely completely re-used in the water-steam circuit.